18.4 Communities, Biomes, and Succession
Key Takeaways
A community is the populations of different species in one place, linked by competition, predation, mutualism, commensalism, and parasitism.
A keystone species has an effect on community structure larger than its abundance suggests.
Primary succession starts on bare rock or new volcanic land with no soil, while secondary succession starts where soil remains and is faster.
Biomes are major terrestrial communities shaped largely by temperature and precipitation, and a desert is defined by low precipitation.
Estuaries, where fresh water meets salt water, are productive nursery habitats, and a climax community is a late stage that can still change.
18.4 Communities, Biomes, and Succession
A community is the populations of different species living and interacting in one place. Structure is that set of species and their interactions. Growth of the community through time is succession. Regulation is what keeps one population from taking all of the space, food, or light.
How species affect one another
Competition occurs when species use the same limited resource, such as food, water, light, or nest sites. Both are held back by the shortage. If the overlap is severe, one species may exclude the other from a habitat, or each may persist at a lower density than it would alone.
Predation is consumption in which the consumer benefits and the organism eaten is harmed. A predator kills and eats an animal. Herbivory is the parallel case, an animal eating a plant. A predator that holds a strong competitor in check can leave room for other species.
Mutualism benefits both species, as with bees and flowers or with fungi and plant roots. Commensalism benefits one species and leaves the other neither helped nor harmed, as when a barnacle rides on a whale. Parasitism benefits the parasite and harms the host, which is usually kept alive. Ticks and tapeworms are parasites.
A keystone species has an effect on community structure larger than its abundance suggests. Sea otters eat sea urchins. Where otters are present, urchins do not strip the kelp, and the kelp forest shelters many other species. The otters need not be the most numerous animals present.
Primary and secondary succession
Succession is the directional change in community composition through time on a new surface or after a disturbance. The starting point decides which kind it is.
Primary succession starts where there is no soil: bare rock, a new lava flow, or land exposed by a retreating glacier. It is not regrowth after a forest fire. Pioneer lichens and mosses cling to rock, trap dust, and add organic matter. Weathering plus that organic matter builds soil, and then grasses, shrubs, and trees can follow. The path is slow because the soil itself has to form.
Secondary succession starts where soil remains. A fire, a hurricane, a flood, or an abandoned field removes or thins the plants without stripping the soil away. Seeds, roots, and soil animals are often still present. Grasses and herbs commonly appear first, then shrubs, then young trees. Because the soil does not have to be manufactured from bare rock, secondary succession is faster than primary succession. Regrowth after a forest fire is this faster path.
A late stage, not a permanent stop
A climax community is the late stage of succession, the community that persists under the local climate until a major disturbance resets it. A temperate deciduous forest is a familiar climax. Real communities keep changing when a tree falls, a storm hits, or a new species arrives. Climax is a late stage, not a permanent stop.
Biomes, set by temperature and precipitation
Biomes are the major terrestrial communities, spread over large regions and shaped largely by temperature and precipitation. Similar climates on different continents produce similar vegetation even when the species are not close relatives. Altitude can imitate latitude. Climbing a tall mountain can pass through communities you would also meet by traveling toward a pole. Name the climate before you name the plants.
| Biome | Climate clue | Biological clue |
|---|---|---|
| Tropical rainforest | Warm all year, with very high rainfall | Huge species diversity and a layered canopy |
| Savanna | Warm, with a wet season and a long dry season | Grasses, scattered trees, and large grazers |
| Desert | Very low precipitation; may be hot or cold | Sparse, drought-adapted plants and animals |
| Chaparral or Mediterranean shrubland | Mild, wet winters and hot, dry summers | Dense woody shrubs, many of them fire-adapted |
| Temperate grassland | Moderate precipitation, hot summers, and cold winters | Grasses, fertile soil, and grazing and burrowing animals |
| Temperate deciduous forest | Four seasons and precipitation through the year | Broadleaf trees that drop their leaves in autumn |
| Taiga or boreal forest | Long, cold winters and short, cool summers | Conifer forests of spruce, fir, and pine |
| Tundra | Very cold, low precipitation, and permafrost | Treeless low shrubs, mosses, and lichens |
A desert is defined by low precipitation, not only by heat. Hot deserts and cold deserts both qualify, and a wet rainforest does not. Chaparral, or Mediterranean shrubland, has mild wet winters, hot dry summers, and many shrubs that recover after fire. Taiga and boreal forest are two names for the northern conifer belt. Tundra is colder still, with permafrost and no tall trees. Tundra also gets little precipitation, but permafrost and low evaporation leave the ground wet, which separates it from a desert.
Aquatic communities
Lakes and ponds are standing fresh water. Light supports rooted plants in the shallows and phytoplankton at the open surface; deeper water depends on food from above. Rivers are flowing fresh water. Near the source the water is often cold, fast, and oxygen-rich. Downstream the current slows and the water carries more sediment.
Wetlands, including marshes, swamps, and bogs, are shallow water over saturated soil. Marshes in particular are productive breeding habitat and they slow floodwater. Estuaries form where a river meets the sea. Fresh water and salt water meet, and these nurseries shelter young fishes and shellfish.
The intertidal zone lies between high and low tide, so organisms are alternately submerged and exposed to drying and waves. Coral reefs grow in warm, shallow, clear seawater. Coral animals house photosynthetic algae, and that mutualism supports very high diversity. The open ocean is vast. Phytoplankton produce food in the sunlit surface, often at a low rate per square meter, and deeper communities depend on food that sinks.
Warning
Primary succession starts on bare rock or new volcanic land with no soil. Regrowth after a forest fire is secondary succession, because the soil is still there, and it is the faster path. A desert is defined by low precipitation, not only by heat.
Two decisions before you name the place
- If there is no soil, the path is primary succession, with pioneer lichens and mosses ahead of larger plants.
- If soil remains, as after a fire or on an abandoned field, the path is secondary succession, and it is faster.
- Temperature and precipitation name the terrestrial biome. A single hot afternoon does not.
- Fresh water standing is a lake or pond. Fresh water flowing is a river. Fresh water meeting salt water is an estuary, a nursery.
Name succession from whether soil is already present. Then name a terrestrial biome from temperature and precipitation.
Grasses colonize a burned forest while the soil is still in place. Which description is correct?
This is primary succession, because a fire always leaves bare rock with no soil.
This is a desert, because the site was hot while the fire burned.
This is secondary succession, which starts where soil remains and is faster than primary succession.
This is primary succession, and it is faster than secondary succession because lichens must build the soil.
Which statement correctly identifies a desert biome?
A desert is defined by low precipitation, and it may be hot or cold.
A desert is an estuary, the nursery where fresh water and salt water mix.
A desert is the first stage of primary succession on bare volcanic rock after a fire.
A desert is any place with high temperature, including a wet tropical rainforest.
Sea otters are not the most numerous animals in a kelp forest, yet without them sea urchins can destroy the kelp. What does that illustrate?
A desert climate defined only by heat, even where rainfall is high.
A keystone species, with an effect on community structure larger than its abundance suggests.
Parasitism, because both the otter and the kelp are harmed by the interaction.
Primary succession on bare rock after a forest fire.
Sections you finish are checked off in the contents.